Provide default value for configuration

[raditya gumay]

As we have seen in the example, the initialization of Courier took few configurations. Since most of them are a default configuration. would be better if we create a bean in the configuration class itself, and the caller is able to mutate as per need. doing this would less overhead for setup the configuration

Example:

data class FooConfiguration(
    val arg1: String,
    val arg2: String,
    val arg3: String
) {

    fun default(): FooConfiguration {
        return FooConfiguration(/*args*/, /*args*/, /*args*/)
    }
}

// client side
val foo = FooConfiguration.default().copy(arg1 = "magic")